I don't think you'll be happy with an LA 40 in a 5 lb. bird. If you looking for 2S options there is always the OS .46 AX (a litte expensive in this size range). Tower sells the SuperTigre GS45 ABC for $64.99. This engine screams with lots of power but it is a little heavy. I have one in a 40 size mustang turning a trimmed MA 11x7 (10.5 x 7). She'll do close to 85 MPH --- very cool on that low high speed pass.

If you looking for a real screamer, the Tower .46 is the most powerful in its class. Its $79.99 but will turn an 10x7 at about 15,000 RPMs.

Another great buy and lots of torque is the SK50. I have one in my Yak-3 40 size. Even though its cheap, its one of my most reliable engines. This engine won't turn more than about 12,000 RPMs, but it will do about 11,200 with an 11x7 ($59.77 from astrcplanes.com).